Treatment of Plastic Waste by Melt Densification- Operational Experience at CWMF

Transcription

1 Available online at Energy Procedia 7 (2011) Asian Nuclear Prospects 2010 Treatment of Plastic Waste by Melt Densification- Operational Experience at CWMF S.V.S. Rao, Biplob Paul, A. G. Shanmugamani, K. Paramasivan and P. K. Sinha Centralised Waste Management Facility, BARC Facilities, Kalpakkam , India Abstract Volume reduction of radioactive solid wastes is carried out with an aim to minimize disposal space requirement. Cellulosic combustible solid wastes like cotton, paper etc. are treated by incineration and the plastic wastes are volume reduced by baling. Compaction of plastic wastes by baling gives low volume reduction factors (3 5) and also the resultant waste bale may spring back to original volume after disposal. With a view to achieve higher volume reduction factors, studies were conducted at CWMF on the melt-densification of plastic wastes at C. The melted plastics were characterized by TGA/DTA & IR Spectroscopy and found that there was no structural loss of the polymer compounds. The melted mass was observed to have the expected theoretical density. A melt densification unit (MDU) was set up at CWMF with a facility for melting 20 kg of the plastic waste in each batch in a 200L MS drum. The system has a furnace with vacuum chamber where plastic waste in drum is melted and an off gas treatment system consisting of a scrubber followed by HEPA filter. A continuous stack monitor is installed to estimate any release of radioactivity in the off-gases. This paper describes the operational experience of melting of different plastic wastes namely polythene sheets & bottles, HDPE pipes and cans, PVC shoe covers and neoprene gloves. About 47M 3 of β-γ and 18 M 3 of α contaminated plastic wastes were melted using the above MDU. The volume reduction factors varied from 2.5 to 30 depending on the initial bulk density of the material. The radioactivity in the off-gases was found to be always below detectable limits. The leach indices of waste matrices were 2 to 3 orders better than the stipulated limits Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of Indra Gandhi Centre of Atomic Research Keywords: waste management; melt densification; leaching 1. Introduction Plastic wastes present formidable problems as they are at present not biodegradable and can resist incineration and in fact incineration may not be possible due to production of toxic fumes. It is also reported (1) that formation of dioxins and furans (cyclic compounds) are favorable when the temperatures are between 300 and 500 C. Further, during the course of incineration fine soot is formed, which may get loaded on the filters along with the radioactivity and this finally leads to the generation of secondary *Corresponding author. T el.: ; fax : address: svsrao@igcar.gov.in Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Indra Gandhi Centre of Atomic Research Open access under CC BY-NC-ND license. doi: /j.egypro

2 S.V.S. Rao et al. / Energy Procedia 7 (2011) wastes. Generally, the plastic wastes in nuclear industry are baled to reduce the volume before their disposal. Compaction of plastic wastes in this way not only results in poor volume reduction factors viz. 3 to 5 and also the waste forms after disposal may spring back to original form occupying larger volume in the trenches. This conditioned form also increases the leaching of radioactivity into the environment. In CWMF, Kalpakkam a method (2) was developed for melt densification of radioactive plastic wastes. A detailed study was carried out on the off-gas analysis during melting process and characterization of the melted specimens for their leach resistance. Thermal and infrared analysis was carried out to find out if there is any structural loss of the of the melted plastics. Based on these studies the melting temperature was optimized between 170 and 180ºC. A furnace was designed with off-gas system for housing a 200L drum with radioactive plastic wastes. This paper describes the melt densification of plastic wastes consisting of polythene sheets & bottles, HDPE pipes & cans, neoprene gloves and PVC shoe covers. 2. Experimental An insulated oven having the internal dimensions of 1800 mm (h) x 1000 mm (b) x 1000 mm (d) was procured for melt densification of plastics. Inside the furnace, a stainless steel vacuum chamber was provided to obtain a vacuum in the furnace better than 50mm WC. A blower with a capacity of 150 CFM was connected to the vacuum chamber to obtain the desired negative pressure. There is an arrangement in the oven for forced circulation of hot air for uniform heating of plastics. There is an arrangement for the treatment of off-gases resulted during heating using an alkali scrubber and HEPA filter bank. The total system is shown in the Figure 1. There are two temperature controllers with adjustable set points. One controller maintains the desired temperature of furnace and the other was kept at 20 C above the set value. Continuous air monitor was connected to find out, if there is any release of radioactivity during the melting. A 200 dm 3 MS drum loaded with plastics was mounted on the trolley and kept inside the oven using a hydraulic lift and the drum was heated in the temperature range of ºC for about 3 hours. Fig.1. Melt Densification unit The temperatures of the vacuum chamber and furnace were measured during the time of heating. The scrubber solutions were analyzed for radioactivity. The off-gases were analyzed for radioactivity, carbon monoxide, hydrochloric acid and hydrocarbons. The densities and the volume reduction factors were computed. To study leaching characteristics polythene matrices containing alpha and beta activity were prepared separately to conduct leaching studies.

3 504 S.V.S. Rao et al. / Energy Procedia 7 (2011) Results & discussion 3.1. Calibration of furnace temperature As the drum was kept inside the vacuum chamber which in turn was placed in the furnace, the temperature of the furnace and vacuum chamber were measured to calibrate the desired furnace temperature. The temperatures observed were plotted against time and are shown in Figure 2. It is observed from the figure that at the beginning i.e. up to 30 minutes the temperature of the vacuum chamber was lower than the furnace temperature. Afterwards the temperature of the vacuum chamber was higher by about 20 C when the constant temperature was observed. The optimized furnace temperature from these experiments was 165 C. Different types of radioactive plastic wastes received from the reactors, reprocessing plant and research laboratories were melted. The volume reduction factors are given in Table 1. As the temperature employed for melting process was low no detectable radioactivity was detected in the off gases. Figure 3. and 4 show the polythene waste packets before and after melting. The density of the matrices were in the range of kg.m 3. The volume reduction factors(vrf) were in the range of 2.5 to 30. High VRF was observed for polythene sheets and low VRF for HDPE pipes because their initial bulk density was high. Fig.2.Temperature Profile of Furnace-Vacuum Chamber (with plastic waste drum-furnace set Temp 1750DegC) During melting, the concentrations of the toxic gases present in the off-gases were measured and their concentrations are given in Table 2. It was observed that the concentrations of the toxic gases were much below the emission limits (3-6). Figure 3 shows the typical polythene waste received from Madras Atomic Power Station(MAPS) and Figure 4 shows the waste in the drum after melting.

4 S.V.S. Rao et al. / Energy Procedia 7 (2011) Type of Plastic Volume Melted (M 3 ) Volume reduction factor" Activity in Offgas (Bq/M 3 ) β α Polythene waste, PVC shoe covers, Neoprene gloves (β,r) BDL BDL Polythene waste, PVC shoe covers, Neoprene gloves (α) BDL BDL Polythene bottles BDL BDL HDPE Pipes BDL BDL HDPE carboys BDL BDL Total Fig.3. Polythene packets before melting Fig.4.Polythene packets after melting 3.2. Leaching studies About 100 g of polyethylene sheets was cut in to small pieces and taken in to a beaker and about 20 ml of 137 Cs having a specific activity of 1.2x10-2 MBq.mL -1 was added. The polyethylene pieces along with radioactivity solution on their surface were allowed to dry for two weeks. In this process the radioactivity got adhered to polyethylene pieces. The whole mass was heated in the oven and the melted polyethylene in the cylindrical form was removed from the container for leaching studies. Similarly alpha containing polythene blocks were prepared by melting the polythene sheet pieces loaded with uranium oxide. For loading, the UO2 dissolved in nitric acid medium was used. After loading the pieces were dried and melted in the mould. About 9 kbq of alpha activity was present in polythene matrix. The leaching studies were carried out with the matrix in accordance with the American Nuclear Society standard leach test method (ANSI 16.1) as reported in the literature (7). The leach rates observed for both the matrices containing alpha and beta & gamma activity are given in

5 506 S.V.S. Rao et al. / Energy Procedia 7 (2011) Figure 5. When equilibrium reached the leach rates were in the order of 10-3 to 10-4 cm/day. The leach indices were computed to be about 8-9 which is 2 to 3 orders better than the stipulated value Fig.5.Leach Rate Vs time 4. Conclusion About 65 M 3 of waste received from various units at Kalpakkam and RMP, Mysore was melt densified without facing any difficulty during the operation. As the temperature of the furnace operation was lower than 200ºC, no change of heating coils or any major ma intenance was needed till date after the MDU commission. As the present MDU has to be operated in a batch process and it is planned to procure a continuous system for future operations. References [1] Ferdinand Engelbeen; Dioxin Formation from Incinerators; org / 1996/ dioxinl/msg00207.html. [2] S.V.S. RAO & K.B.Lal; Melt Densification of Plastic Wastes; Journal of Radioanalytical and nuclear chemistry, Vol.270(2006) [3]US EPA 40 CFR Part 503 Rule; Chapter 17,Incinerator Emissions; [4]29CFR, Part , OSHA, US [5]Central Pollution Control Board (CPCB), New Delhi, [6] bcairquality/reports/ecmswi.html [7]A.A.Moghissi, H.W.Godbee, and S.A. Hobart; Radioactive Waste Technology, The American Society of Mechanical Engineers; New York (1986).